Attributing trends in extremely hot days to changes in atmospheric dynamics

Abstract

Abstract. This paper presents a method for attributing regional trends in the frequency of extremely hot days (EHDs) to changes in the frequency of the atmospheric patterns that characterize such extraordinary events. The study is applied to mainland Spain and the Balearic Islands for the extended summers of the period 1958-2008, where significant and positive trends in maximum temperature (T x ) have been reported during the second half of the past century.First, the study area was split into eight regions attending to their different temporal variability of the daily T x series obtained from the Spain02 gridded data set using a clustering procedure. Second, the large-scale atmospheric situations causing EHDs are defined by circulation types (CTs). The obtainment of the CTs differs from the majority of CT classifications proposed in the literature. It is based on regional series and on a previous characterization of the main atmospheric situations obtained using only some days classified as extremes in the different regions. Three different atmospheric fields (SLP, T850, and Z500) from ECMWF reanalysis and analysis data and combinations of them (SLP-T850, SLP-Z500, and T850-Z500) are used to produce six different CT classifications. Subsequently, links between EHD occurrence in the different regions and CT for all days have been established. Finally, a simple model to relate the trends in EHDs for each region to the changes in the CT frequency appearance has been formulated.Most regions present positive and significant trends in the occurrence of EHDs. The CT classifications using two variables perform better. In particular, SLP-T850 is the best for characterizing the atmospheric situations leading to EHD occurrences for most of the regions. Only a small number of CTs have significant trends in their frequency and are associated with high efficiency causing EHD occurrences in most regions simultaneously, especially in the northern and central regions. Attribution results show that changes in circulation can only explain some part of the regional EHD trends. The percentage of the trend attributable to changes in atmospheric dynamics varies from 15 to 50 %, depends on the region and is sensitive to the selected large-scale variables.

Weather Risk Assessment for Collective Water Supply and Sewerage Systems

“…However, regional studies show some variation in results. The upward trends of hot days occurrence have been shown in most studies, among others for the area of Switzerland [98], Turkey [99], and Spain [100], while there are countries for which no statistically significant trend of changes was recorded, incl. for New Zealand [101], or even a slight downward trend in the number of hot days, as in China [102].…”

Assessing space and time changes in daily maximum temperature in the Ebro basin (Spain) using model‐based statistical tools

“…The slope of these extents is , more than double the slope of the extents based on daily temperatures. In addition to the increasing trend, this plot permits us to identify years which were colder with respect to the trend, and with a lower variability, for example, 1972, 1977, 1984, or hotter as year 2003 (García‐Valero et al, 2015). In the last decade, two different types of behaviours are observed, the distribution of the extent in some years is quite high, centred around 0.9, while others centred around 0.5.…”

Recent Progress and Emerging Topics on Weather and Climate Extremes Since the Fifth Assessment Report of the Intergovernmental Panel on Climate Change

“…For instance, Horton et al (88) attributed much (∼75%) of the recent increases in cold extremes over central Asia to a robust increase in "ridge-trough" pairs (see 88, figures 4b,f ), reasonably explaining the phenomena deviated from the prevalent warming backdrop. García-Valero et al (89) reported a fairly high contribution (∼50%) of changes in circulation patterns in determining trends of hot extremes in Spain and the Balearic Islands. Chang et al (90) revealed that the weakening activity of extratropical cyclones led to the warming of maximum temperature in the mid-latitudes in Northern Hemisphere.…”